ABSTRACT The majority of HIV-infected adolescents (84%) live in sub-Saharan Africa, and Kenya ranks among the top five countries with the highest burden of adolescents living with HIV (ALHIV). ALHIV have lower rates of retention, adherence and viral suppression than children and adults. As ALHIV transition to independent adult HIV care, they are at high risk of disengaging with care. Improving the transition process, including disclosure and independent medical management, is critical to improve ALHIV outcomes. High volume HIV clinics require feasible, effective, and systematic approaches to improve transition, including translation of evidence-based interventions. In partnership with the Kenyan Ministry of Health and partner organizations leading HIV care provision in Kenya, our team has adapted evidence-based interventions supporting transition and combined them into an Adolescent Transition Package (ATP). ATP effectiveness (in improving retention and viral suppression) is being tested in an RCT (NIH R01HD089850-01; PI John-Stewart) with RCT initiation in mid-2019. The proposed administrative supplement will improve measurement of implementation context that can inform effectiveness interpretation and future sustainability. We propose to expand the RCT to a hybrid implementation-effectiveness trial by including evaluation and analysis of key implementation outcomes necessary for optimizing ATP delivery and for future scale-up, including: intervention acceptability, feasibility, adoption, penetration, and fidelity. In the 10 clinics receiving the intervention, we will evaluate how facility-led ATP adaptations influence early (6 month) implementation outcomes through structured quality improvement activities with healthcare providers. We will use mixed methods to identify determinants influencing late-stage implementation outcomes including adoption, penetration and fidelity at the end of the clinical trial. We will assess associations of implementation outcomes with ATP effectiveness in the parent trial. We will also collect cost data and evaluate cost-effectiveness of the ATP. Simultaneous evaluation of implementation determinants and outcomes is an efficient and timely approach to optimize interpretation of study results and provide uniquely valuable information for future scale-up of the ATP.